Amplifier for microwaves comprising radial waveguide-hybrid coupler

ABSTRACT

A radial waveguide-3dB hybrid coupler in a circular configuration comprises two coaxial radial wave-guides, one with an input port and one with an output port, having a branch-guide hybrid coupler formed in the common broad wall. Dual arrays of one-port, reflective type, power amplifying modules are mounted at the periphery of the waveguide structure. Input power is coupled to both module arrays and reflected to the output port to provide gain-enhanced output power over broad bandwidths.

United States Patent 1 [11] 3,775,694

Quine Nov. 27, 1973 [5 AMPLIFIER FOR MICROWAVES 3,733,560 5/1973 Ohman, Jr. etal. 3330/34 COMPRISING RADIAL WAVEGUIDE-HYBRID COUPLER John P. Quine, Colonie, N.Y.

Assignee: General Electric Company,

Schenectady, NY.

Filed: Feb. 2, 1973 Appl. No.: 329,234

Inventor:

US. Cl 330/56, 331/46, 331/96, 333/10, 333/24 R, 333/98 M Int. Cl. H03f 3/60 Field of Search 333/10, 24 R, 98 R, 333/98 M; 330/34, 56', 331/96 References Cited UNITED STATES PATENTS 6/1971 Hines 330/34 X POWER AMPLlFlER MODULE Primary ExaminerPaul L. Gensler Attorney-Donald R. Campbell et a1.

[ 5 7 ABSTRACT A radial waveguide-3dB hybrid coupler in a circular configuration comprises two coaxial radial waveguides, one with an input port and one with an output port, having a branch-guide hybrid coupler formed in the common broad wall. Dual arrays of one-port, reflective type, power amplifying modules are mounted at the periphery of the waveguide structure. Input power is coupled to both module arrays and reflected to the output port to provide gain-enhanced output power over broad bandwidths.

9 Claims, 4 Drawing Figures MODE 13' b 5 ABSORBER PAIENIED 3.775.694

sum 1 OF 2 POWER AMPLIFIER MODULE PATENTED NOV 2 7 I973 SHEET 2 0F 2 AMPLIFIER FOR MICROWAVES COMPRISING RADIAL WAVEGUIDE-HYBRID COUPLER BACKGROUND OF THE INVENTION This invention relates to microwave power combiners, and more particularly to a radial waveguide-hybrid coupler structure for combining the power of a plurality of reflective-type solid state modules each having gain.

The power from a large number of microwave amplifiers and oscillators can be combined in suitable radial waveguide structures with high combining efficiency, stability, and broad bandwidth. It is feasible to combine the power from a hundred or more sub-combiner modules or single elementary power devices arranged about the periphery of the radial waveguide. A simple radial waveguide power combiner has a single input-output port at the center coupled to a circulator, such that input power excites cylindrical waves that travel outwardly to the modules and are reflected back towards the center with power gain. An important fault which limits its use to narrow-band applications arises from the fact that the input and output lines are not isolated from one another. A relatively small reflection at the output line transition or circulator produces a large effect on the bandpass characteristics of the power combiner. This difficulty is caused by the relatively long distance between the output line or circulator and the subcombiner modules, and is known as the long-line effect.

To eliminate the problem of the long-line effect associated with the one-port circular reflector configuration, a two-port configuration having separate input and output ports can be empolyed such as the two focal points of a waveguide that is elliptical rather than circular. Power radiated from the input focus is reflected from the modules along the elliptical surface and is refocused at the other focal point. An isolator is needed at the input port to absorb the gain-enhanced reflection from the output connector or load. The focal point separation should be small as compared to the major and minor axis dimensions to obtain relatively equal module gain and phase. Other disadvantages of elliptical waveguide power combiners are that the dimensions of the input and output coupling connectors, which ideally act as infinitely small probes, must be kept small to minimize energy scattering, and that higher order mode absorption slots can be placed only along the major axis without affecting the desired mode.

SUMMARY OF THE INVENTION In accordance with the invention, a radial waveguidehybrid coupler power combiner for X-band and other microwave frequencies comprises first and second radial waveguides, one with an input port and the other with an output port. The radial waveguides are typically circular in configuration, coaxially mounted with a common central plate or broad wall, and have coaxial transimssion line to waveguide transitions at the centrally located input and output ports. An array of power amplifying modules are mounted about the periphery of each radial waveguide. The module is suitably a oneport, reflective type, solid state module containing an elementary power device or a multiplicity of power devices (a sub-combiner module) and is operative to receive incident waves and produce gain-enhanced reflected waves. A hybrid coupler, preferably a 3dB branch-guide, connects the first and second radial waveguides and functions to couple input power applied to the input port to the power amplifying modules in both radial waveguides, and to couple the gainenhancd reflected waves produced by both module arrays to the output port. Advantageously the 3dB hybrid coupler is comprised by a plurality of concentric annular rings supported approximately in the plane of the common broad wall between the two coaxial radial waveguides. Optionally, the outer walls of the radial waveguides have a series of radial slots, and microwave absorption material is secured to the outer surfaces to absorb unwanted higher order modes. Among other advantages, the radial waveguide -3dB hybrid coupler is easy to fabricate and is useful in high power, broad bandwidth applications.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diametrical cross-sectional view through the radial waveguide-3dB hybrid coupler microwave power combiner constructed in accordance with the invention;

FIG. 2 is a cross-sectional view taken approximately on the line 22 of FIG. 1 showing in plan the central plate or common wall and hybrid coupler;

FIG. 3 is a cross-sectional view taken approximately on the line 3-3 of FIG. 1 showing an outer waveguide plate with radial higher order mode absorption slots; and

FIG. 4 is a schematic, enlarged, diametrical crosssectional view of a modification of the hybrid coupler and dual radial waveguide illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the new microwave power combiner comprises a pair of coaxially mounted circular radial waveguides indicated generally at 10 and 11 that preferably have a conductive common broad wall or plate 12. The lower radial waveguide 10 has a parallel conductive outer wall 13 with a central circular aperture for mounting the outer conductor of an input coaxial transmission line 14. The central conductor may or may not be connected to the common plate 12, but is illustrated in contact. The low reflection coaxial transmission line to waveguide transition serves as the input port 15, and other suitable low reflection transitions or junctions can be employed. In similar fashion, the upper radial waveguide 11 has a central circular aperture in parallel conductive outer wall 13 for mounting theouter conductor of an output coaxial transmission line 16, the central conductor also being connected to the common broad wall 12. This coaxial transmission line to waveguide transition serves as the output port 17. Each of the radial waveguides l0 and l 1 has the same radius R and height b, thereby forming similar parallel-plate regions. Input power applied to the input port 15 excites an outwardly travelling circular transverse electromagnetic (TEM) wave in the lower parallel-plate region, with an axially directed electric field E as shown.

At the periphery of each of the radial waveguides 10 and 11 is mounted an array of microwave power amplifying modules M that are preferably identical to one another, or approximately identical, and cover a full 360 arc (see FIG. 2). The power amplifying modules M more particularly are one-port reflective type modules, each containing an array of solid state microwave active devices or a single elementary solid state microwave power device. Typical solid state power devices that are suitable are transistors, transferred-electron diodes, and avalanche diodes. A module containing a series or parallel array of these devices is commonly known as a sub-combiner module. Any appropriate one-port reflective type module, as is known in the art, that receives incident waves and produces gainenhanced reflected waves can be used in the practice of the invention. For example, the module can use a rectangular waveguide with a single input-output port that faces towards the center of the radial waveguide structure. Another arrangement can use modules coupled by means of magnetic loop probes or electric field probes. The details of structure and operation of such microwave power amplifying modules are well-known in the art and need not be further described or illus trated. A large number of these modules, for example 100, are arranged to form a one-dimensional H-plane array radiating directly into the parallel plate region with the E-vector perpendicular to the plates. When properly designed, there are no holes in the H-plane radiation pattern.

An essential element of the dual waveguide structure is a 3dB hybrid coupler, indicated generally at 18, for coupling incident waves to the power amplifying modules M in both radial waveguides and, in turn, to couple the gain-enhanced reflected spherical waves from both arrays of modules to the output port 17 to provide output power. An advantage of the -3dB hybrid coupler 18 is that it is contained entirely within the parallelplate system and is constructed to be supported approximately in the plane of the common plate or broad wall 12. Hybrid coupler 18 is a specially designed form of branch-guide coupler comprising a plurality of spaced, concentric, annular conductive rings l8a-l8d that effectively provide a series of circular slots in the common wall between the two parallel-plate regions. Four annular rings are illustrated here, but as many as ten can be used if desired. As shown in FIG. 2, rings 18a-18d can be supported, as for example, by a plurality of radial wires 19 connected at the ends of the common plate 12 that bridge the branch-guide apertures at intervals of about 0.8K in the circumferential direction. Other fabrication techniques are also possible and can be economically implemented. This type of branchguide hybrid coupler gives high isolation and excellent balance over bandwidths greater than 10 percent.

In the lower radial waveguide 10 (FIG. 1), the input port to 3dB hybrid coupler 18 is identified as port 1 and the port adjacent modules M as port 3, while in the upper radial waveguide 11 the output port is identified as port 2 and the port adjacent the module array as port 4. As is further explained with regard to FIG. 4, the spacing of the circular slot centers is about M4 and the thickness of the annular rings is also about M4 For this configuration, the input power applied at the input port 15 by means of coaxial transmission line 14 excites a cylindrical TEM wave that is incident on port 1 of the -3dB hybrid coupler 18. Power then divides nearly evenly between ports 3 and 4. The arrays of power amplifying modules M are connected to both ports 3 and 4, and the incident waves experience power gain upon reflection from the modules. The reflected waves from the modules have the correct relative phases, specifically a 90 phase shift between the upper and lower modules, to cause nearly all of the power to be transmitted to port 2 of the hybrid coupler. This power then passes to the output port 17 of the dual radial waveguide structure. Only a small portion of the reflected wave energy appears at input port 1 and passes to the input port 15. The coaxial transmission line to waveguide transition, it will be recalled, is a low reflection transition. By way of further explanation, it can be shown by mathematical analysis that the reflection on port 1 is small when the complex voltage reflection gains of the individual modules connected to ports 3 and 4 are nearly equal. For high gain, a small difference between the two complex voltage reflection gains, as can be expected in individual cases because of the expected statistical differences between a single module at port 3 and a single module at port 4, could result in a sizable reflection on port 1. However, the use of a module array comprised of a large number of modules connected to ports 3 and 4 tends to reduce the statistical difference between the two complex voltage reflection gains. Therefore, the reflection on port 1 of the -3dB hybrid coupler will be relatively small.

Any non-uniformities in the single-device or subcombiner modules M set up higher order waves in the parallel-plate regions which do not couple to the output coaxial transmission line 16 (or to the input coaxial transmission line 14). These higher order waves must be absorbed in order to avoid resonances within the parallelplate regions. With the circular radial waveguide configuration, this is accomplished simply by providing the outer plates 13' and 13, respectively, with a series of radial slots 20 and 20 as illustrated in FIGS. 3 and 2. The undesired higher order modes have circumferential current components that are coupled through the radial slots to metal-enclosed regions containing a suitable mode absorber material 21 (FIG. 1). This mode absorbing structure is built onto the outside surfaces of each of the plates 13' and 13. The desired mode having no circumferential current component anywhere in the parallel-plate regions is unaffected by the radial slots 20 and 20'.

In the modification illustrated in FIG. 4, the dual radial waveguide structure has a relatively thick common broad wall plate 12 with the result that the two arrays of modules for the coaxial radial waveguides 10 and 11 can be axially spaced from one another. Within the broader scope of the invention, the thick common wall plate 12 can be replaced by axially separated separate plates. These plates are commonly made of aluminum or brass. In the diagrammatic representation of the 3dB hybrid coupler 18, only three of the annular rings l8a-l8c are illustrated although any number of branches can be empolyed. The centerline spacing S of the indiviual branch waveguides, and also the length L, are both made equal to about )\/4. The height H of the individual branch waveguides, corresponding to the width of the circular slots, can vary. In the operation of such a branch waveguide, incident power at port 1 splits and is partially transmitted toward port 3 and partially coupled through the branch waveguide to upper radial waveguide 11 where a portion of the energy travels outwardly and a portion inwardly. This action is repeated at each of the other branch waveguides. In radial waveguide 11, the radially inwardly directed energy destructively interferes. Within the broader scope of the invention, other types of hybrid couplers can be employed.

The radial waveguide 3dB hybrid coupler is advantageous for S-band to X-band frequencies, but can be used at other microwave frequencies. It is feasible to combine the power from a hundred or more subcombiner modules in this way. When each module contains l diodes, each capable of supplying five watts of average power, the total combined average power is 5,000 watts. This power combiner has the desirable characteristics of high combining efficiency, stability, and broad bandwidth. By way of example, a coupling unbalance of less than 0.2dB can be obtained between ports 3 and 4 over greater than 18 percent bandwidth. The reflection in input port 1 and the coupling to port 2 are below 30dB over this band. Another advantage is the ease of fabrication.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A microwave power combiner comprising a first radial waveguide having an input port and a second radial waveguide having an output port,

a plurality of power amplifying modules mounted about the periphery of each of said radial waveguides, each module receiving incident waves and producing gain-enhanced reflected waves, and

hybrid coupling means connecting said first and second radial waveguides and operative to couple input power applied to said input port to said power amplifying modules in both waveguides, and to couple the gain-enhanced reflected waves produced by said power amplifying modules in both waveguides to said output port to provide output power.

2. A construction according to claim 1 wherein said hybrid coupling means is comprised by a branch-guide hybrid coupler.

3. A construction according to claim 1 wherein said first and second radial waveguides are coaxially mounted circular waveguides with a common conductive wall.

4. A construction according to claim 3 wherein said hybrid coupling means is comprised by a branch-guide hybrid coupler.

5. A construction according to claim 3 wherein said hybrid coupling means is a branch-guide hybrid coupler comprised by a plurality of concentric annular conducting rings supported approximately in the plane of said common wall.

6. A construction according to claim 2 wherein said first and second radial waveguides each has an outer conductive wall with a plurality of radial higher order mode absorption slots, and microwave absorption material secured to the outer surfaces of said outer walls.

7. A microwave power combiner comprising a pair of coaxially mounted circular radial waveguides having a common conductive wall, one of said radial waveguides having a central input port and the other a central output port,

an array of one-port power amplifying modules mounted about the entire periphery of each of said radial waveguides, each module receiving incident waves and producing gain-enhanced reflected waves, and

a hybrid coupler comprising a plurality of spaced annular conducting rings supported approximately in the plane of said common wall that are operative to couple input power applied to said input port to said power amplifying modules in both waveguides, and to couple the gain-enhanced reflected waves produced by said power amplifying modules in both waveguides to said output port to provide output power.

8. A construction according to claim 7 wherein said circular radial waveguides each have a conductive outer wall with a plurality of radial higher order mode absorption slots, and microwave absorption material secured to the outer surfaces of said outer walls.

9. A construction according to claim 8 further including coaxial transmission line to waveguide transitions at said input and output ports. 

1. A microwave power combiner comprising a first radial waveguide having an input port and a second radial waveguide having an output port, a plurality of power amplifying modules mounted about the periphery of each of said radial waveguides, each module receiving incident waves and producing gain-enhanced reflected waves, and hybrid coupling means connecting said first and second radial waveguides and operative to couple input power applied to said input port to said power amplifying modules in both waveguides, and to couple the gain-enhanced reflected waves produced by said power amplifying modules in both waveguides to said output port to provide output power.
 2. A construction according to claim 1 wherein said hybrid coupling means is comprised by a branch-guide hybrid coupler.
 3. A construction according to claim 1 wherein said first and second radial waveguides are coaxially mounted circular waveguides with a common conductive wall.
 4. A construction according to claim 3 wherein said hybrid coupling means is comprised by a branch-guide hybrid coupler.
 5. A construction according to claim 3 wherein said hybrid coupling means is a branch-guide hybrid coupler comprised by a plurality of concentric annular conducting rings supported approximately in the plane of said common wall.
 6. A construction according to claim 2 wherein said first and second radial waveguides each has an outer conductive wall with a plurality of radial higher order mode absorption slots, and microwave absorption material secured to the outer surfaces of said outer walls.
 7. A microwave power combiner comprising a pair of coaxially mounted circular radial waveguides having a common conductive wall, one of said radial waveguides having a central input port and the other a central output port, an array of one-port power amplifying modules mounted about the entire periphery of each of said radial waveguides, each module receiving incident waves and producing gain-enhanced reflected waves, and a hybrid coupler comprising a plurality of spaced annular conducting rings supported approximately in the plane of said common wall that are operative to couple input power applied to said input port to said power amplifying modules in both waveguides, and to couple the gain-enhanced reflected waves produced by said power amplifying modules in both waveguides to said output port to provide output power.
 8. A construction according to claim 7 wherein said circular radial waveguides each have a conductive outer wall with a plurality of radial higher order mode absorption slots, and microwave absorption material secured to the outer surfaces of said outer walls.
 9. A construction according to claim 8 further including coaxial transmission line to waveguide transitions at said input and output ports. 